DESCRIPTION: One of the most serious side effects associated with the therapy of HIV-1 infection is the appearance of viral strains that exhibit resistance to protease inhibitors. The main goal of this project is to understand the molecular mechanisms by which specific mutations in the HIV- 1 protease elicit inhibitor resistance and to incorporate that knowledge in the development of new strategies for drug design. A related goal is to understand the effectiveness of inhibitors against protease molecules from different HIV- 1 subtypes. These proteases contain amino acid variations at several locations, including some that have been associated with drug resistance. The main questions that this project will answer are: 1) How do specific mutations in the protease molecule preferentially lower the binding affinity of inhibitors relative to the substrate? What are the thermodynamic and structural basis of resistance? 2) How effective are current inhibitors against proteases from different HIV- 1 subtypes? While subtype B is the predominant in the United States and has been the target for drug design, other subtypes are prevalent in Africa, Asia and some European countries, and account for most of the HIV-l infections worldwide. 3) Is the thermodynamic origin of the forces that define the binding affinity of inhibitors related to their susceptibility to resistance-causing mutations? Is it possible to identify protease inhibitors with drastically different enthalpy/entropy balances but similar affinities? Do they respond to resistance-causing mutations in a different way? 4) Is it possible to develop molecular design strategies that explicitly consider the susceptibility to resistance causing mutations? These questions will be answered by a combination of experimental thermodynamic measurements (high sensitivity isothermal titration calorimetry and high sensitivity differential scanning calorimetry), structure determination (x-ray crystallography) and structure-based thermodynamic calculations.